1. Field of the Invention
The present invention relates to a semiconductor module for stacking, which is disposed in a lowermost layer when a plurality of semiconductor devices are stacked together, and to a stacked semiconductor module using the same.
2. Description of the Related Art
With demands for miniaturization and improved functionality of electronic devices of various kinds, such as portable telephone devices or digital still cameras, stacked semiconductor modules (package on package) have been developed, in which electronic components, in particular, a plurality of semiconductor devices and chips are stacked together and united in a single body.
In mounting of such a stacked semiconductor module, a connection yield between upper and lower packages due to ever higher density and ah improvement in inspection sensitivity in line with higher functionality have become an issue, and in response to this, an inspection yield has been required to be improved by improving the quality.
Japanese Patent Application Laid-Open No. 2004-363126 and the like disclose a conventional stacked semiconductor module. FIG. 8A shows the conventional stacked semiconductor module, in which a second package PK2 is stacked and mounted on a first package PK1.
External connection terminals 2 are provided on the lower surface of the first package PK1 and upper/lower connection terminals 1 are provided on the upper surface of the first package PK1. In a normal product, the upper/lower connection terminals 1 are bonded onto upper layer module connection pads 15a formed on the upper surface of the first package PK1. Furthermore, the shape of the connection surface of the upper/lower connection terminal 1 and the shape of the external connection terminal 2 are both a similar circular shape. Moreover, the upper layer module connection pads 15a also have a circular shape, as shown in FIG. 8B.
Japanese Patent Application Laid-Open No. 2000-208557 describes improved connection reliability between a printed substrate and a semiconductor device by forming lands 4 provided on a substrate 11 of the semiconductor device in a rectangular, elliptical or oval shape, or the like, so as to be elongated in the direction of thermal expansion, as shown in FIG. 9.
Normally, an electrical inspection is used to judge connections between upper and lower semiconductor devices. Specifically, generally a re-inspection is carried out on a completed stacked product after connection of the upper and lower devices, in order to check the connection status between the upper and lower devices and ensure electrical properties.
In this inspection, the stacked semiconductor module is inserted into a socket of an inspection apparatus, the external connection terminals 2 of the first package PK1 are brought into contact with a probe on the socket side by applying a load from above, and the electrical conduction in the connecting portion between the first package PK1 and the second package PK2 is judged via the internal wiring of the stacked semiconductor module.
However, in such an electrical inspection, if the bonding state between the first package PK1 and the second package PK2 is not reliable and it is not certain whether or not the packages are simply in contact with each other, an electrically conducting contact may be achieved temporarily in the connecting portion between the first package PK1 and the second package PK2 due to the load applied during the inspection, and the product may be judged as “electrically satisfactory” and pass the inspection.
Therefore, in order to judge whether or not a sound physical bond is formed between the upper and lower connection terminals and electrical connection is maintained therebetween, rather than the upper and lower connection terminals simply contacting each other, an X-ray inspection or ultrasonic inspection (SAT: Scanning Acoustic Tomograph) is used to perform a non-contact transmissive inspection.
However, in such a transmissive inspection, visual judgment requires quite a long time, and automatic judgment makes it difficult to set a threshold value, so that it is extremely difficult to judge defects with respect to whether or not a bond has been made.
Specifically, in the example shown in FIGS. 8A and 8B, since the shape of the pads 15a is circular, and the external connection terminals 2 also have a circular ball shape, the pads and external connection terminals both produce the same circular shape even when the stacked semiconductor module is observed by transmission from above in the non-contact transmissive inspection. Moreover, while the connection surface presents a circular shape when the upper and lower terminals are in a bonded state, even if the upper and lower terminals are in an unbonded state or a non-contact state, the planar shape of the ball of the upper terminal itself is visible and presents the same shape. Since connection terminals of the same shape such as the lowermost terminals and the upper terminals are used in combination, regardless of their size, then even in an unbonded state, the size and shape are obviously the same as in a bonded state, and since the shape is the same as that of the other connection terminals, then it is difficult to distinguish bonded locations and the other connection terminals.
As shown in FIG. 9, Japanese Patent Application Laid-Open No. 2000-208557 discloses a technology aimed at improving the connection strength of the semiconductor device with respect to the printed substrate by forming the lands 4 in a rectangular shape. However, this technology clearly does not address the issue of the inspection in which terminals are present in upper and lower positions in a stacked state as in the stacked semiconductor module.
Specifically, since differences with respect to the upper and lower connection terminals and differences in shape in the respective connecting portions are not made clear simply by using the rectangular lands as seen in Japanese Patent Application Laid-Open No. 2000-208557 in the upper and lower connection terminals, then in the transmissive inspection using X-rays or the like, there is a possibility that connection terminals of the same shape, such as the lowermost terminals and the terminals between the upper and lower devices, are mixed.